1. Field of the Invention
The present invention relates to an exhaust gas recirculation apparatus and particularly to an exhaust gas recirculation apparatus in which improvements of control characteristics of an electrically controlled valve are made.
2. Description of the Related Art
An exhaust gas recirculation apparatus (hereinafter referred to as “EGR apparatus”) is a well known apparatus that feeds a part of exhaust gas back to an intake system, lowers the maximum temperature in combustion of mixed gas, and thus restrains the quantity of generated NOx, in order to reduce nitrogen oxide in the exhaust gas.
In such an EGR apparatus, a valve is usually provided at the position where a part of the exhaust gas is fed back to the intake system and its opening is adjusted to adjust the quantity of feedback of the exhaust gas to the intake system. As such a valve, typically, an electrically operated so-called electromagnetic valve is preferably used in view of ease and the like due to its electric control. Many apparatuses of such structures have been proposed (for example, see JP-A-2002-285888).
Meanwhile, when an electromagnetic valve is used for adjusting the quantity of feedback of the exhaust gas in the EGR apparatus, there is a problem that the temperature characteristics of the electromagnetic valve are affected by the control accuracy of the quantity of feedback. That is, an electromagnetic coil forming the electromagnetic valve typically has a resistance component and this resistance component varies depending on the ambient temperature and the temperature of the electromagnetic coil itself. As a result, the energizing current changes and this affects the valve opening, that is, the quantity of feedback of the exhaust gas. Such a problem can similarly occur in a motor-driven valve.
As a solution to such current changes in the electromagnetic coil, for example, a technique of detecting the actual current value and changing the quantity of energization to the original value by feedback control may be considered.
As a solution without using feedback control, for example, a technique of finding an estimate value of coil temperature by a predetermined arithmetic expression and adjusting the quantity of energization to the coil in accordance with the estimate value is well known (for example, see JP-A-2001-214766).
However, for example, in a situation where a circuit for such feedback control cannot be newly added because of sharing or the like of an energizing circuit for the electromagnetic coil, it may be demanded to restrain current changes in the electromagnetic coil without using feedback control based on current value detection. The above-described former technique is not necessarily reliable.
In the case of the above-described latter technique, the current correction is solely based on the estimate value of the coil temperature. The technique is insufficient if there are other factors that cause changes in the current flowing through the electromagnetic coil.